Numerous studies have implicated a serotonergic mechanism in the CNS regulation of prolactin release. Previous data from this laboratory (Barofsky and Harney, Neuroendocrinology 26: 333, 1978) have demonstrated impairments in lactation in the rat following electrolytic lesions of the median raphe nucleus, a major midbrain locus of serotonergic cell bodies projecting to the hypothalamus. The current studies were designed to examine the role of the serotonergic neurons of the midbrain raphe nuclei in lactation and prolactin release. Lactation was assessed by measuring litter growth rates and the release of prolactin following the application of a standard 30-minute suckling stimulus on day 7 of lactation in animals with specific serotonin neurotoxin lesion (5,7-dihydroxytryptamine) in the median or dorsal raphe nuclei. In addition, endogenous metabolic activity of the raphe nuclei during suckling was measured using the 2-deoxyglucose metabolic mapping procedure.